With the rapid increase of LNG regasification facilities in Europe and North America, LNG traders are directing their export focus to these countries in addition to various Asian countries such as Japan, Korea, and China. While most Asian countries prefer a high Btu content natural gas, North American pipeline specification restricts the import to low Btu value content gases, for emission control reasons. Hence, in traditional LNG liquefaction plants, NGL removal is limited to C5 and heavier hydrocarbons to avoid plugging of the cryogenic exchanger, and most of the lighter NGL components are liquefied together with the methane component, resulting in LNG with a fairly high Btu content. When such LNG is exported to North America or Europe, deep removal of the NGL components is typically necessary prior to LNG liquefaction, in order to meet the relatively low heating value specification, ranging from 960 Btu/scf to 1100 Btu/scf.
Alternatively, the rich LNG when imported to the LNG regasification terminals, can be diluted with nitrogen, or blended with a leaner natural gas to lower its heating value or Wobbe Index. However, there are upper limits on the amount of nitrogen and inerts that can be introduced to the pipeline gas, and in most cases, a lean gas source is not readily available. Moreover, dilution with nitrogen requires an air separation plant to produce the nitrogen, which is energy intensive and costly and produces no environmental benefit.
Therefore, to compete in the LNG export markets, LNG liquefaction plants must be provided with the flexibility to produce different heating value LNG for export to different customers. This means the LNG liquefaction plants are required to add an NGL recovery unit for the removal of the lighter NGL components when exported to Europe or North America. While the cost of these NGL recovery units may be justified for larger LNG plants, it is often not economical for smaller LNG plants, particularly when retrofitting existing LNG plants.
There are numerous configurations and methods known in the art for high recovery of C3+ components from a natural gas feed. However, all these known processes are complex and costly. Some of the NGL/LNG integrated examples include the expander processes described in U.S. Pat. No. 4,157,904 to Campbell et al., U.S. Pat. No.4,251,249 to Gulsby, U.S. Pat. No. 4,617,039 to Buck, U.S. Pat. No. 4,690,702 to Paradowski et al., U.S. Pat. No. 5,275,005 to Campbell et al., U.S. Pat. No. 5,799,507 to Wilkinson et al., and U.S. Pat. No. 5,890,378 to Rambo et al. Other C3+ recovery methods are also known, as exemplified by U.S. Pat. No. 6,308,531 to Roberts et al, where a side stream from the cryogenic exchanger is processed in a scrub column for the removal of the heavier hydrocarbons. These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
While these processes can achieve heating value reduction to at least some extent, removal of C3+ components is limited, especially at high pressure (e.g., 700 psig and greater) where separation of C3+ components from C2 and lighter components is difficult. Consequently, when processing a rich gas with a high C2 content (e.g., 10% and higher), these processes will often require excessive refrigeration and may no longer be economical.
In still further known configurations, as described for example in U.S. App. No. 2007/0157663, an NGL recovery unit provides a low-temperature and high-pressure overhead product directly to the LNG liquefaction unit and feed gas cooling and condensation are performed using refrigeration cycles that employ refrigerants other than the demethanizer/absorber overhead product. Thus, the cold demethanizer/absorber overhead product is compressed and delivered to the liquefaction unit at significantly lower temperature and higher pressure without net compression energy expenditure. While such systems and methods provide certain advantages, various drawbacks nevertheless remain. Among other things, external refrigeration may become cost-prohibitive, and operational flexibility is often not readily implementable.
Thus, while numerous plant configurations and methods for NGL recovery and LNG liquefaction are known in the art, all or almost all of them, suffer from various disadvantages. Thus, there is still a need for improved NGL recovery and LNG liquefaction, and especially plants in which NGL recovery and LNG liquefaction are integrated.